This invention relates to methods and apparatus for confining a liquid sample for thermal analysis. It is particularly useful for confining of volatile liquids in photocalorimetric analysis.
Thermal analysis techniques such are highly useful in studying chemical reactions within a liquid. Photocalorimetric analysis, for example, measures the thermal response of a sample upon irradiation with light. Photocalorimetric analysis is widely used in the study of photo-induced polymerization.
Ideally the liquid sample under study should be confined in such a fashion that it cannot escape by evaporation. For photocalormetric analysis, the sample should also be optically accessible in a thin, uniform layer. Evaporation loss and thickness variation would introduce error.
The only commercially available sample containers that can handle volatile samples for thermal analysis have metal covers or lids. They are therefore unsuitable for photocalorimetry because light cannot penetrate through the metal cover. Commercially available sample containers intended for photocalorimetry cannot contain volatile liquids. They consist of an aluminum pan and a quartz window that sits on the pan. The quartz window allows light to penetrate and irradiate the sample; however it does not provide a good seal, so volatile samples can partly or totally evaporate during the measurement. Evaporation is undesirable because the change in mass and the heat of vaporization are sources of error.
A second difficulty with commercially available sample containers is that they often present liquids (or solidified liquids) in films of nonuniform thickness. In order to obtain good results, the thickness of the sample should be uniform. But liquids typically wet the surface of the sample container and form a meniscus. The thickness in the center of the container is smaller than thickness at the walls. Accordingly there is a need for improved methods and apparatus for confining liquid samples in thermal analyses.
In accordance with the invention, a liquid sample for thermal analysis is disposed within a receptacle having a bottom surface and side walls. The top edges of the side walls are bent towards the center of the receptacle. A sheet of flexible, transparent material substantially impermeable to the sample is disposed across the top edges of the side walls, and an open lid compresses an o-ring onto the sheet material, sealing it against the bent top edges of the receptacle. The bottom surface of the receptacle is advantageously coated with a a material not wetted by the sample such as a fluorcarbon.